The present invention relates to a switching power supply apparatus and, more particularly, to an improvement in a power supply circuit for preventing an inrush current upon supply of power to an input terminal of the switching power supply apparatus and transmitting a signal representing completion of charging of an input filter capacitor to a control section.
A conventional power supply circuit of this type used for a switching power supply apparatus has a circuit arrangement based on a relay.
A conventional circuit will be described below with reference to FIGS. 4 and 5. As shown in FIG. 4, a switching power supply apparatus is constituted by a DC/DC converter 26 and a power supply circuit 28. The power supply circuit 28 comprises resistors 13 to 18, a capacitor 19, npn transistors 20 and 21, a relay 22, and a diode 24. Reference numeral 25 denotes a DC power supply; and 27, a load. When a stepped input voltage is applied to an input terminal (point e) of the switching power supply apparatus, a voltage waveform obtained by differentiating the stepped input voltage appears at a point g in FIG. 4, as shown in FIG. 5. With this operation, since the npn transistor 21 is turned on, and the base potential of the npn transistor 20 is pulled down to the negative input, the npn transistor 20 and the relay 22 are in an OFF state. The voltage at the point g reaches its peak when the input voltage is applied, and is gradually decreased. When this voltage becomes lower than the base-emitter voltage V.sub.VE of the npn transistor, the npn transistor 21 is turned off, and the npn transistor 20 is turned on to drive the relay 22. As a result, the negative terminal of the capacitor 19 is connected to the negative power supply input terminal through a contact 23 of the relay 22. At the same time, a signal representing the completion of charging of the input filter capacitor is transmitted to a signal input terminal S.sub.IN of the DC/DC converter 26. FIG. 5 shows voltage waveforms at the point e, a point f, the point g, and a point h in a state wherein the power supply circuit shown in FIG. 4 is operated.
As described above, in the prior art, the characteristics of a power supply circuit are determined by the sensitivity voltage of a relay to be used. For this reason, in a case wherein a switching power supply has a considerably different input voltage condition, the relay must be replaced with a relay having different sensitivity voltage characteristics. In addition, if an input voltage varies in a wide range, the operation margin of a relay is reduced at the lower limit value of the input voltage, and the operation of the power supply circuit becomes unstable, resulting in poor versatility.